athk: import common code for ath1?k drivers

Import common ISC-licensed athk parts assumed to be
based on Linux kvalo/ath.git master at
6bae9de622d3ef4805aba40e763eb4b0975c4f6d.

The only modification should be for FreeBSD module
handling in main.c.

Add the module build framework unconnected to the
build for now.

These files will be shared by ath1?k drivers.

MFC after: 2 months